Philippe Lehoux
complete
You can now manually un-merge the emails in a conversation by individually un-merge them in new conversations, and re-merge these new conversations.
Debra Taylor
Perhaps the ability to "undo" the merge could somehow be integrated at the bottom of the conversation (where it lists all the actions that have occurred), and it could give an option (to the right or something) to undo that particular action, which would in turn, undo any related actions thereafter as well.